This invention relates to a radiation resistant optical waveguide fiber. In particular, the invention relates to a radiation resistant optical waveguide fiber which is doped with fluorine or drawn at low tension.
The broadband capacity, dielectric properties and light weight of waveguide fiber makes it desirable for communication, data and sensing uses in military, nuclear reactor and space applications. To meet the requirements of these applications, considerable work has been done to develop a radiation resistant waveguide fiber.
The term radiation resistant, as applied to an optical waveguide, usually refers to the resistance of an optical waveguide to attenuation increase (induced attenuation) due to exposure to x-rays, gamma rays, neutrons or the like. However, radiation resistance may also refer to rate of decrease in induced attenuation after radiation exposure has ceased or to the total time required for the induced attenuation to fall to zero, or nearly to zero, after exposure has ceased. Thus, in general, radiation resistance may be defined in terms of induced attenuation, rate of recovery or time to full or partial recovery. Each of these measures of radiation resistance may be important in waveguide fiber applications where radiation is present. The term radiation resistance will, in general, be used in reference to these three measures of radiation resistance.
The measured radiation resistance of an optical waveguide fiber depends upon several variables including:
--amount and type of dopants in the fiber;
--temperature of the fiber during and after exposure;
--hydroxyl ion content of the fiber;
--interstitial hydrogen content of the fiber;
--type of radiation incident upon the fiber;
--radiation dose rate;
--total radiation dose; and
--number of separate radiation exposures experienced by the fiber.
Combining these factors with the definition of radiation resistance above, radiation resistance of a waveguide fiber may be expressed in terms of:
--total attenuation induced by a given radiation type, a given radiation dose and a given radiation dose rate at a given temperature;
--a plot of attenuation vs. time which shows the characteristic recovery, i.e., decrease in induced attenuation, after radiation exposure has ceased; and,
--time interval, after exposure has ceased, to recover to a given multiple of the pre-irradiation attenuation.
Proper evaluation of the effect on radiation resistance of a change in waveguide structure or manufacturing method, in general, requires that a reference waveguide fiber be provided as a standard of comparison. The reference waveguide fiber is essentially identical to the test fiber in terms of composition, dimensions, refractive index profile and manufacturing method. Except for the exposure to radiation, the reference fiber is maintained in essentially the same environment as a test fiber and measured each time the test fiber is measured.
Radiation of sufficiently high energy incident on a waveguide fiber is believed to break or ionize bonds (produce defects) in the glass waveguide fiber structure. These defects can absorb or scatter light traversing the waveguide thereby increasing the waveguide attenuation. Further, it is believed a waveguide fiber which has defects in bond structure is more susceptible to ionizing radiation and is more likely to retain structural damage done by ionizing radiation. Hence, efforts to produce a radiation resistant waveguide fiber have been directed to:
i) producing a fiber from materials which form strong bonds;
ii) producing a fiber which is low in defect concentration.
Y. Hibino, et al., "Formation of Drawing-Induced E' Centers in Silica Optical Fibers," Japanese Journal of Applied Physics, 1985, reported increase in the E' defect centers with increasing draw temperature and with increasing draw speed.
The reported results do not appear to relate draw tension to the formation of E' defects. Increased temperature usually implies lower draw tension. And increased draw speed usually implies higher draw tension. Yet both increased draw speed and increased temperature result in a greater number of E' centers.
The work of H. Hanafusa, et al., "Drawing Condition Dependence of Radiation-Induced Loss in Optical Fibers", Electronics Letters, V. 22, No. 2, 1986, showed recovery of the irradiated fiber, one day after exposure, worsened as fiber draw speed decreased from 14.3 m/sec to 2 m/sec. However, recovery improved as fiber draw temperature increased from 2000.degree. to 2255.degree. C.
Here again the data is not consistent with regard to predicting improved radiation resistance under low or high draw tension conditions.
The work of Lyons, et al., "Influence of Preform and Draw Conditions on UV Transmission and Transient Radiation Sensitivity of an Optical Fiber", Proceedings SPIE, Vol. 1174, Fiber Optics Reliability: Benign and Adverse Environments III, 1989 Symposium, dealt with transient response of fiber attenuation irradiated with high energy x-ray pulses, Co.sup.60 and UV light. Draw tension was varied between 12 and 185 grams (96 and 1480 dynes/cm.sup.2 for a 125 micron O.D. fiber). The transient response was reported to be improved with lower draw tension. However, the data does not consistently show that low draw tension provided improved radiation resistance. In their conclusion statement, low draw tension is not recognized as a key variable in producing radiation resistant fiber. "Very low draw speeds, even though they minimize draw tension, do not provide optimized UV and Co.sup.60 performance independent of the fiber buffer material . . . . Intermediate draw speeds provide optimum performance." (p. 18, para. 4. ) Also draw tensions below 12 grams (96 dynes/cm.sup.2) were not investigated.
Askins, et al., reported the effect on long term recovery of draw tension in the range 20 to 80 grams (160 and 640 dynes/cm.sup.2 for a 125 micron O.D. fiber). No correlation was reported. These workers did report, "at some tension higher than 50 grams, the initial loss (measured immediately after irradiation) increases sharply." No systematic investigation of the effect of low draw tension was reported.
U.S. Pat. No. 4,988,162, Hayami, relates to a radiation resistant multiple fiber, wherein a number of optical fiber elements each of which comprises a core composed of a pure silica glass and a cladding layer formed on the core and composed of a doped silica glass are mutually heat fused, and said pure silica glass of the core having a chlorine content of lower than 1 ppm, an OH group content of lower than 1000 ppm, and also fluorine content of at least 100 ppm." (Col. 1, II. 3845.)
The limitations of this structure are:
i) no co-dopant in the core restricts the number of attainable shapes of the index profile; and,
ii) higher OH' ion concentration results in higher waveguide attenuation at wavelengths near the characteristic OH' absorption peaks.
The attainable profiles are also limited in U.S. Pat. No. 5,163,987, Ishiguro et al., which relates to, "a method for producing a glass preform for use in the fabrication of an optical fiber, in which fluorine is homogeneously added." (Col. 2, II. 42-45.)
U.S. Pat. No. 5,146,534, Lines, is directed to a structure which incorporates certain alkali metals and fluorine in the core. "The alkalis that can produce such loss [intrinsic attenuation] reductions are Na, K and Rb. I have also discovered that co-doping with F can significantly extend the concentration range in which doping with K and Rb can yield loss reduction." (Col. 2, II. 23-27.)
Here again the specified structure is limited in the index profiles it can produce. Also, the addition of alkali metals to the core may not be expected to improve radiation resistance.
U.S. Pat. No. 5,210,816, lino et al., is directed to a waveguide fiber, ". . . characterized in that at least the core portion is doped with fluorine and an oxide and in that the fluorine contained in the core portion is doped more toward the center in the radial direction than the outer circumferencial portion." col. 1, II. 66-68, col. 2, II. 1-2.
The fluorine is used to yield a step index profile in light of a processing phenomenon in which the core oxide diffuses out of the core during a preform processing step. See col. 1, II. 39-47. The fluorine doping is non-uniform to compensate for the non-uniformity of the core oxide diffusion.
The prior art does not define drawing parameters which consistently improve radiation resistance regardless of the waveguide fiber composition. Also, the effect of co-doping with fluorine has not been fully explored and understood. Furthermore, potential advantageous interaction between fluorine co-doping and drawing conditions has not been thoroughly studied.